I wrote a quick plugin that adds native Prowl support for notifications to Indigo. Using this plugin, you don't need to have Growl installed on your system: it utilizes the Prowl API directly.
Prowl is a simple app that allows you to send arbitrary notifications to your iOS devices. By installing it for Indigo, you'll be able to send push notifications as actions. It does require a one-time purchase of $4 (at the time of this writing) to get the app. After installing the app on your device, notifications are first processed by Prowl and then able to redirect to Indigo (note that I haven't implemented this feature yet). You can also try out sending push notifications from the Prowl website to get a sense for how the system works.
You will need to have a Prowl API key for your application. These are free to generate on the Prowl site. I would recommend creating a specific key for this application. After installing the plugin, you will be prompted to enter a valid API key to enable Prowl support.
I did look into Pushover support, which also looks very nice. Prowl was really quick to get up and running, so I'm giving it a shot.
You can download the plugin here. Current version is 0.2.3.
The source for the plugin is hosted on GitHub. Feel free to check it out. I've also created a release page if you'd like to see the official release.
After installing, you will be able to select "Prowl Notify" from the notifications menu in your actions. Let me know if you have any questions / issues! I'm still experimenting with this, so it's unclear how far I'll take it...